Ultrasonic imaging has been applied in many two dimensional systems using pulse echo B-mode tomography or B-scans. These systems display echoes returning to an ultrasonic transducer as brightness levels proportional to echo amplitude. The brightness levels may be used to create cross-sectional images of the object in the plane perpendicular to the transducer aperture.
Examination of objects in three dimensions has evolved using a number of modalities including x-ray, ultrasound, and nuclear magnetic resonance. In particular, improvements have been made in spatial resolution, dynamic range, display methods and data analysis. For example, ultrasound scanning of three-dimensional objects by sequential B-scans followed by off-line reconstruction and display of rendered images has progressed in recent years with the introduction of commercial three-dimensional systems. Off-line rendering, however, may take several minutes to produce a single three-dimensional scan.
In the area of high-speed three-dimensional ultrasound imaging, U.S. Pat. No. 4,596,145 to Smith and von Ramm discloses an acoustic imaging system capable of producing high-speed projection orthoscopic images, as well as a single high-speed C-scan image using a two-dimensional array transducer and receive mode parallel processing. The C-scan image may be defined as a planar section of the object parallel to the effective transducer aperture. In 1987, U.S. Pat. No. 4,694,434 to von Ramm and Smith disclosed a steered array acoustic imaging scanner capable of producing a high-speed pyramidal scan to obtain a volumetric (three-dimensional) image using a two-dimensional array transducer and receive mode parallel processing.
High frequency intraluminal ultrasound imaging probes have been developed, including circular arrays and mechanically steered transducers. The circular arrays and mechanically steered transducers produce B-mode circular side scan geometries in which the ultrasound beam is swept through a 360.degree. arc. The 360.degree. arc may create a high-speed circular image within a vessel or lumen with a maximum range of approximately one centimeter. For example, U.S. Pat. No. 3,938,502 to Bom and U.S. Pat. No. 4,917,097 to Proudian, et al. disclose circular arrays of transducer elements within a catheter to produce a circular side scanning intraluminal B-mode image. U.S. Pat. No. 4,794,931 to Yock and U.S. Pat. No. 5,243,988 to Sieben, et al. disclose motor-driven piston transducers at the end of the catheters to produce circular side scanning intervascular imaging.
Catheters may be used in conjunction with the systems described above to provide intraluminal imaging. Intraluminal imaging may involve inserting a catheter, that includes an ultrasonic transducer phased array, into coronary vessels, pulmonary arteries, the aorta, or venous structures. For example, U.S. Pat. No. 5,704,361 to Seward, et al. discloses a volumetric imaging ultrasound transducer under-fluid catheter system. The advantages of Seward may, however, be limited by the quality of the imaging provided therein. In particular, the catheter probes disclosed in Seward show the therapeutic tools adjacent to the transducer array on the catheter tip, thereby reducing the area available for the transducer array. Such an array may provide images having reduced spatial resolution. Moreover, the applications described in Seward may be limited to procedures involving catheters.
The catheters described above may be combined with electrodes or tools to locate (cardiac electrophysiological mapping) and perform therapy on (radiofrequency ablation) or monitor tissue. For example, a three-dimensional ultrasound imaging device using a catheter may be combined with an ablation electrode to provide therapy to particular tissue. The therapy provided by the electrode, however, may be limited by the registration between the image provided by the catheter and the electrodes associated with the catheter. For example, a user may have difficulty translating the image produced by the catheter to the position of the electrode, thereby possibly creating difficulty in applying the electrode to the intended tissue. Moreover, the electrode may obscure the three dimensional ultrasound image when the electrode is within the field of view of the image.
In view of the above discussion, there exists a need to improve the quality of real-time three-dimensional imaging in intraluminal ultrasound applications.